In order to address a reduction in communication quality attributed to a sharp recent increase in mobile traffic and to provide faster high-speed communication, the 3GPP LTE (Long Term Evolution) has examined standardization of the carrier aggregation (Carrier Aggregation: CA) function in which a radio base station (eNode B: eNB) communicates with a radio terminal (User Equipment: UE) using a plurality of cells. Note that, cells that a UE can use in CA are limited to cells of a single eNB (i.e., cells operated by a single eNB).
The CA procedure will be described with reference to FIG. 17 (Non Patent Literature 1). FIG. 17 shows an example in which a UE performs CA of a first cell (Cell1) and a second cell (Cell2) both operated by an eNB. In Step S1, the UE establishes a radio connection with the eNB on the first cell (RRC Connection Establishment). In Step S2, the UE receives downlink data from the eNB through the first cell (Downlink data on Cell1). Here, for the UE, the first cell is the primary cell (Primary cell: PCell).
In Step S3, the eNB determines that it is necessary to configure a secondary cell (Secondary cell: SCell) for the UE, and configures a second cell as the SCell through the PCell (RRC Connection Reconfiguration on Cell1 (including Configuration of Cell2 (Secondary cell: SCell)). In Step S4, the UE sends a completion notification to the eNB in response to completion of configuration of the second cell, that is, in response to completion of preparation of using the second cell (RRC Connection Reconfiguration Complete).
In Step S5, the eNB sends to the UE a notification of second cell use initiation (Cell2 Activation). In Step S6, the UE receives downlink data by simultaneously using the first and second cells (DL data on Cell1 and Cell2). Note that, the UE in Step S6 is just required to be capable of simultaneously using the first and second cells for downlink data reception. In other words, the UE is not required to constantly receive downlink data on both the first and second cells. Whether to use one of the first and second cells for downlink data reception or to use both of them is determined, for example, based on downlink data quantity or a service that the UE uses. The case where the UE transmits uplink data on the second cell can also be basically carried out by using a procedure similar to that shown in FIG. 17.
Though the UE that performs CA has functions of a Physical layer and at least part of a MAC (Medium Access Control) layer for each of the aggregated cells, the UE has the same structure of the RLC (Radio Link Control) and higher layers as in the case where CA is not performed. Accordingly, the core network (Evolved Packet Core: EPC) does not know whether or not the UE is performing CA.
Further, about a heterogeneous network (HetNet) environment, there is proposed a concept of Inter-eNB CA in which cells operated by different eNBs are aggregated (Non Patent Literature 2). For example, in Inter-eNB CA, it is considered to use a macro cell operated by a macro base station (Macro eNB: MeNB) and a pico cell operated by a pico base station (Pico eNB: PeNB).
Still further, there is also proposed a method in which a macro cell with wide coverage is used for transmission and reception of signals related to control such as UE mobility management, and a pico cell which has relatively good communication quality is used for transmission and reception of signals related to data such as user data (Non Patent Literature 3).